[imcc] Fix one more -O2 bug #1041 get_results write

correctly detect get_results writes to a reg.
skip constant propagation then.

There was an old wrong in instruction_writes for exception handling
before get_results. exceptions are now proper PMCs.

Also add more debug prints for -d41 in constant propagation.

Fixes all testO2 testcases but 3:
t/op/exit.t                       (Wstat: 256 Tests: 9 Failed: 1)
  Failed test:  6
  Non-zero exit status: 1
t/op/jitn.t                       (Wstat: 256 Tests: 14 Failed: 1)
  Failed test:  13
  Non-zero exit status: 1
t/pmc/exception-old.t             (Wstat: 256 Tests: 22 Failed: 1)
  Failed test:  19
  Non-zero exit status: 1
ChangeLog: add GH #1041 summary